This invention relates to making halftone copies of continuous tone documents.
In modern graphics processing it is desirable to substitute electro-optical systems for the process camera contact screening systems historically used to make halftones. One example of such an electro-otpical system is disclosed in Behane U.S. Pat. No. 3,604,846. In that system the original document is scanned to produce digital video signals each of which is scaled in accordance with the tone density in an incremental area of the original corresponding to a cell of the desired halftone screening pattern. The video signal is then used to control the number of marks made in each halftone cell, to create the effect of variable sized halftone dots. In Moe U.S. Pat. No. 3,646,262 the original document is similarly scanned at a resolution equal to the density of the desired screen to generate a video signal for each cell area, and a recording beam is shaped in accordance with the video to produce variable sized halftone dots. In Hennig U.S. Pat. No. 3,671,666 the desired halftone screening pattern and the original document are scanned smultaneously, at a resolution higher than that of the screen density, the video and screen signals are combined, and the halftone output is made black or white depending upon whether the combined signal is above or below a pre-set threshold value. By way of background the disclosures of the above patents are incorporated herein by reference.